1. Field of the Invention
The invention relates to a sawing strip and to a method carried out using this sawing strip for simultaneously cutting off a multiplicity of slices from a cylindrical workpiece, in particular a workpiece consisting of semiconductor material, the workpiece and a wire frame of a wire saw performing, with the aid of a feeding device, a relative movement directed perpendicular to the longitudinal axis of the workpiece, by which the workpiece is guided through the wire frame.
2. Background Art
Semiconductor wafers are generally produced by a cylindrical, monocrystalline or polycrystalline workpiece of the semiconductor material being divided up into a multiplicity of semiconductor wafers simultaneously in one operation with the aid of a wire saw.
The main components of these wire saws include a machine frame, a feeding device and a sawing tool, which comprises a frame made up of parallel portions of wire. The workpiece is fixed on what is known as a sawing strip, generally by cementing or gluing it on. The sawing strip is in turn secured on a mounting plate, in order to clamp the workpiece in the wire saw. Various types of sawing strips are disclosed in U.S. Pat. No. 6,035,845. The sawing strips according to the prior art are distinguished by a substantially rectangular cross section, one side of the sawing strip being adapted to the cylindrical form of the workpiece by a concave curvature.
The wire frame of the wire saw is generally formed by a multiplicity of parallel portions of wire, which are clamped between at least two wire guiding rollers, the wire guiding rollers being rotatably mounted and at least one of them being driven. The portions of wire generally belong to a single, endless wire, which is guided spirally around the roller system and is unwound from a supply roller onto a take-up roller.
During the sawing operation, the feeding device brings about an oppositely directed relative movement of the portions of wire and of the workpiece. As a consequence of this feeding movement, the wire, to which a sawing suspension is applied, works its way through the workpiece, forming parallel sawing gaps. The sawing suspension, which is also referred to as slurry, contains particles of hard material, for example of silicon carbide, which are suspended in a liquid. A sawing wire with fixedly bound particles of hard material may also be used. In this case, application of a sawing suspension is not necessary. All that is needed is to add a liquid cooling lubricant, which protects the wire and the workpiece from overheating and at the same time transports slivers of workpiece out from the cutting gaps and away.
The production of semiconductor wafers from cylindrical semiconductor material, for example from single crystal ingots, places high requirements on the sawing method. It is generally the aim of the sawing method that each sawn semiconductor wafer has two faces that are as planar as possible and lie parallel to each other.
A part from the variation in thickness, the planarity of the two faces of the semiconductor wafer is of great significance. After the dividing up of a semiconductor single crystal, for example a silicon single crystal, by means of a wire saw, the wafers produced as a result have a wavy surface. In the subsequent steps, such as for example grinding or lapping, this waviness can be partially or completely removed, depending on the wave length and amplitude of the waviness and on the depth of the material removal. In the worst case, even after polishing, remnants of this waviness may still be detected on the finished semiconductor wafer, where they have adverse effects on the local geometry. These waves are present to varying degrees at different locations on the sawn wafer. Particularly critical is the end region of the cut, where particularly pronounced waves can occur and, depending on the kind of steps that follow, may also be detectable on the end product.
It is known from DE 102005007312 A1 that the wave in the end region of the cut that occurs in sawing processes according to the prior art is particularly pronounced in the case of slices which have been cut off from the ends of the cylindrical workpiece. In the middle of the workpiece (in the axial direction) on the other hand, the cut-off slices have virtually no waves in the end region of the cut. Furthermore, the axial back pressure gradient produced by the sawing suspension was identified as a cause of the wave produced at the end of the sawing process. According to DE 102005007312 A1, therefore, the amount of sawing suspension applied to the wire frame is reduced, and as a result the waviness of the sawn semiconductor wafers in the end region of the cut is reduced. However, it has been found that this measure is not adequate to satisfy the increasing requirements for the local geometry.